This invention relates generally to a cryogenic refrigeration system and cryostat, and more particularly to a cryogenic system operating within a range of temperatures from 70K to 120K, using mixed refrigerants in a closed circuit including a vapor compressor, a counter flow heat exchanger, and a throttle device to provide refrigeration effect for a low temperature evaporator.
Where a refrigerating system is intended to provide very low temperatures in the cryogenic range, such as between 70K and 120K, the refrigerants comprise cryogenic gases usually having boiling temperatures below 125K, such as nitrogen, which has a normal boiling temperature of 77K at one standard atmospheric pressure, or argon, which has a normal boiling temperature of 87K, or methane which has a normal boiling temperature of 112K. These cryogenic gases have typically required the use of very high pressure gas systems involving specially designed multi-stage compressors. Such systems are expensive to manufacture and operate, and require frequent maintenance.
In order to provide cryogenic systems which are less costly and more efficient, numerous mixed gas refrigerants have been proposed for use within the cryogenic temperature range. These mixed refrigerants typically combine standard well known cryogenic refrigerant components such as nitrogen, argon or methane, and also include additional components such as ethane, propane or isobutane, in various combinations. Each of these mixtures provides a specific arrangement of components with specified percentages of the various ingredients.
However, while significant improvements have been made by using mixed refrigerants, problems still exist with the multi-component refrigerant mixtures.
For example, steady state operation of a cryostat generally provides a constant thermal load on the refrigeration system. Once attained, the desired steady state temperatures are maintained when operating within the design capacity of the compressor. However, during a transient thermal load condition on the system, such as during cool-down from room temperature to the desired steady operating temperature, the rapidity of cool-down has been limited. Intermediate cooling steps are sometimes provided for accelerating cool-down before the steady-state vapor compressor cycle takes over. Still, there is a wide range of cooling capacity requirements and rapid cool-down requires modification of the system between the cool-down and steady-state modes.
In closed cycle vapor compression systems for cryogenic cooling, lower supply pressures, i.e., compressor discharge pressure, are used to permit the use of a single stage compressor. However, such low pressure refrigerators have problems that were not apparent with earlier operations at higher pressures. For example, FIGS. 1 and 2 show refrigeration cycle efficiencies using refrigerant mixtures that have been recommended for operation at compressor supply pressures above 5 Mpa. Efficiencies are greatly reduced if the same refrigerant mixture is used in a system operating at 2.5 Mpa and below at compressor discharge. Mixtures 1, 2 and 3 in FIGS. 1 and 2 represent mixtures of basic components nitrogen, methane, ethane and propane in various proportions as disclosed in British patent No. 1336892 to Alfeev et al. (Nov. 1993).
A further problem in working at compressor discharge pressures less than 2.5 Mpa is that mixtures of the prior art require a variable restrictor device to achieve cool-down from ambient temperatures to steady state cryogenic refrigerating temperatures in a minimum time. It is necessary to adjust the restrictor setting so as to reduce refrigerant flow and achieve lower operating temperature during steady state, as compared to the restrictor setting for reasonably fast cool-down.
Therefore, it has been necessary in the prior art, wherever practical experience has made operators knowledgeable of the problem, to use an adjustable throttle device. Then, at one setting of the throttle device cool-down is effected, and at a different setting of the throttle device that provides a greater refrigerant flow restriction, steady state operation is effected.
Generally, in prior art publications, the problem of disparate thermal load is not recognized and is not addressed. Many cycles described and illustrated in such publications on a predictive basis, in actuality will be inoperative or impractical if an adjustable expansion device is absent, because of the extended time period which will be necessary to cool the cryostat to the desired temperature before steady state operation commences.
Throttle devices are generally made adjustable in capacity by providing a flow path that is variable in flow resistance, e.g., the flow area is varied when an orifice is used. To accomplish this variability, throttle devices have been fabricated of materials with different coefficients of thermal expansion such that there is relative motion between elements, which varies flow area, as the operating temperature of the throttle device drops. Thus, the throttle device becomes complicated. The need for an automatic mechanical device to track a changing thermal load reliably and rapidly come to rest at the desired steady state operating conditions, provides many difficulties in constructing an automatic expansion device.
Externally adjustable devices, e.g., manual, may also be used. However, precision control of orifice size is difficult and heat leakage problems are exacerbated when control elements must be made accessible outside the cryostat. Separate restrictors, one for cool-down and another for steady state operation, have also been used.
What is needed is a refrigeration system for a cryostat that takes advantage of refrigerant mixtures to improve efficiency, and accommodates both cool-down and steady state operating loads using a fixed throttle device, such as an orifice or capillary tube, which has no moving parts.